As part of a search for biological activity in compounds containing strong electron-withdrawing groups directly attached to C=(), a series of derivatives of perfluoroalkyl aldehydes and perfluoroalkyl acids were subjected to biological evaluation. Interesting and diverse activities were found in derivatives of trifluoroacetaldehyde: in contrast, derivatives of longer chain fluorinated aldehydes and acids showed little activity. The active compounds which, it is suggested, may act by releasing CFjCHO in the body of the test animal have effects on the endocrine glands and the CNS and reduce inflammation induced by carrageenin. Koine of these show potent, but temporary, antifertility activity in male rats.Carbonyl derivative* of o-membered heterocyclic rings, substituted with strong electron-withdrawing groups, have been the subject of numerous publicat ions.1